1. Field of the Invention
The present invention relates to a method and an apparatus for setting the gap distance between two objects at a predetermined distance. More specifically, the invention relates to a method and an apparatus for setting the gap distance between a mask and a wafer at a predetermined distance in parallel relation when the image of a circuit pattern of the mask is transferred to the wafer in a semiconductor manufacturing process.
2. Description of the Related Art
In the process of manufacturing a semiconductor device, such as a VLSI, a circuit pattern of the device, which is previously formed on a mask, is transferred to a wafer. In an X-ray exposure apparatus, for example, X-rays are applied to the wafer through the mask having the circuit pattern, whereby an image of the pattern is transferred to the wafer. Before the transfer of the circuit pattern, the mask and the wafer, which are opposed to each other, should be arranged so that the gap distance between them is set at a predetermined distance, ranging from 20 to 50 .mu.m, for example.
Conventionally known is a method for setting the gap distance between the mask and the wafer on the basis of the focusing effect of lenses.
In this method, three CCD cameras, whose optical axes extend parallel to one another, are arranged at the back of a mask chuck. Each camera is movable along its own optical axis. A mark is formed on each of the film of the mask and the wafer so as to be situated on the optical axis of each camera. One of the CCD cameras is moved from a position where it is focused on its corresponding mark of the mask or the wafer to a position where it is focused on the mark of the other. Since the distance between the two focusing positions of each CCD camera is equal to a gap distance between the mask and the wafer with respect to one point, the latter can be obtained by measuring the former. Likewise, gap distances between the mask and the wafer can be obtained with respect to two other points by means of the two other CCD cameras. By adjusting these gap distances, the mask and the wafer can be set parallel to each other so that the gap distance between them is fixed at a predetermined distance.
According to the method described above, however, it is relatively difficult to discriminate the focusing positions, so that the gap distance between the mask and the wafer cannot be accurately detected. Therefore, setting the gap distance cannot be accomplished with accuracy. Since a complicated optical system is need to attain the focusing effect of the lenses, the construction of an apparatus for setting the gap distance is not very simple. Since the measurable range with respect to the direction of the gap distance is relatively narrow, furthermore, the mechanical mounting accuracy of the mask and the wafer and their thicknesses should be accurately controlled.
Also known is a conventional method in which diffraction gratings are used to set the gap distance. According to this method, however, ripples are superposed on detection signals, so that the gap distance cannot be accurately detected. Since a complicated optical system is required for the measurement of the gap distance, moreover, the apparatus for setting the gap distance has a complicated construction.
If the accuracy of the gap distance setting means used is relatively low as aforesaid, the mask and the wafer, which face each other across the very fine gap distance, may possibly run against each other. It is not advisable, therefore, to use such means. When the gap distance is detected at the respective central portions of the mask and the wafer, in particular, the possibility of their running against each other is very high, since a substantial error is caused at their respective peripheral portions, although the distance at the central portions is subject to only a minor error.